Metal-organic frameworks (MOFs) are a relatively new class of porous materials that are comprised of metal ion/oxide secondary building units (SBUs) interconnected by organic linking ligands. MOFs are characterized by low densities, high internal surface areas, and uniformly sized pores and channels. For example, U.S. Pat. No. 8,653,292 describes Zr MOFs having a surface area of at least 1020 m2/g or, if functionalized, having a surface area of at least 500 m2/g. As a result of these advantageous properties, MOFs have been investigated extensively for applications in gas separation and storage, sensing, catalysts, drug delivery, and waste remediation. The wide array of potential applications for MOFs stem from the nearly infinite combination of organic ligands and secondary building units available. Regardless of this diversity, many materials have been left undiscovered due to limitations in the synthetic protocols typically employed for MOF synthesis. The relatively high temperatures and long crystallization times employed to synthesize metal-organic frameworks preclude the incorporation of sensitive moieties. Furthermore, the multiple conformations possible between the ligand and metal SBUs make predicting and directing structure challenging.
Recently a novel method of accessing new MOF materials from a starting host framework has been realized through the use of post-synthetic linker and ion exchange. This method, referred to in the literature as Solvent Assisted Ligand Exchange (SALE) or Post-Synthetic Exchange (PSE), is discussed by, for example, Karagiaridi, O.; Bury, W.; Mondloch, J. E.; Hupp, J. T.; Farha, O. K. in “Solvent-Assisted Linker Exchange: An Alternative to the De Novo Synthesis of Unattainable Metal-Organic Frameworks”, Angew. Chem. Int. Ed. 2014, 53, 4530-4540. This technique has allowed for the development of novel materials which have thus far eluded researchers.
One example of the SALE process is disclosed in U.S. Pat. No. 8,920,541 using a species of MOF known as a zeolitic imidazolate framework or ZIF, as the host framework. In particular, the '541 patent discloses a method for exchanging the imidazolate linker in a zeolitic imidazolate framework composition, said method comprising the steps of: (a) providing a first zeolitic imidazolate framework composition having a tetrahedral framework comprising a general structure, M1-IMa-M2, wherein M1 and M2 comprise the same or different metal cations, and wherein IMa is an imidazolate or a substituted imidazolate linking moiety; (b) providing a liquid composition comprising IMb, wherein IMb is an imidazolate or a substituted imidazolate which is different from IMa; and (c) contacting the first zeolitic imidazolate framework composition with the liquid composition under conditions sufficient to exchange at least a portion of IMa with at least a portion of IMb and to produce a second zeolitic imidazolate framework composition, M1-IMc-M2, wherein IMc comprises IMb, and wherein the framework type of the second zeolitic imidazolate framework composition is different from the framework type obtained when a zeolitic imidazolate framework composition is prepared by crystallizing a liquid reaction mixture comprising a solution of M1, M2 and IMb. One notable result of this work was the complete exchange of 2-methylimidazole (mim) in ZIF-8 for 5-azabenzimidazole (5-abim) to isolate a novel ZIF framework, EMM-19, composed of 5-abim linkers connected to zinc tetrahedra in a sodalite (SOD) topology. This particular structure had been hypothesized to be unobtainable due to the propensity of azabenzimidazole linkers to form ZIFs with LTA-type topologies. This discovery allowed for the development of materials with highly desirable CO2 adsorption characteristics not observed in the nearly identical ZIF-7.
Another type of relevant post-synthetic transformation is Solvent Assisted Ligand Incorporation (SALI), in which functional moieties are grafted onto the ligands and/or secondary building units of MOFs. For example, Hupp and coworkers demonstrated that the treatment of the Zr-based framework, NU-1000, results in the dehydration and grafting of pendant carboxylate and phosphonate moieties onto the secondary building unit (See MOF Functionalization via Solvent-Assisted Ligand Incorporation: Phosphonates vs Carboxylates. Inorg. Chem. 2015, 54, 2185-2192 and Perfluoroalkane Functionalization of NU-1000 via Solvent-Assisted Ligand Incorporation: Synthesis and CO2 Adsorption Studies. J. Am. Chem. Soc. 2013, 135, 16801-16804. Interestingly, this transformation occurs without loss of crystallinity of the parent material and serves to tune the adsorption properties of the resulting material.
Despite these advances, there remains a need for new methods of post-synthesis modification of MOF structures and particularly for those methods which allow the production of ligand/SBU combinations that are difficult or impossible to access by conventional MOF synthesis routes.